Continuously variable transmissions have been in use for years, mainly where horsepower requirements are relatively low, such as in snowmobiles and lawn mowers. Because of the advantages they offer in fuel economy over that of power gear transmissions, inroads into the automotive market have been predicted for some time. Although still the object of extensive development projects, continuously variable transmissions have not lived up to their promise in the automotive market. Efforts to develop a traction drive, which transmits power from one rolling element to another, have not been successful due to problems with low durability and limited power capabilities. Belt and pulley drive systems offer more hope, but still face difficult problems.
In a belt and pulley drive, the belt moves around a pair of variable diameter pulleys, one of which is mounted on the drive shaft and the other of which is mounted on the driven shaft. Each pulley consists of two pulley halves, one of which is mounted for axial movement toward and away from the other. When one of the split pulleys moves apart, the belt rides further down in the V-shaped space between the conical faces of the two halves. At the same time the other split pulley closes, causing the belt to ride higher in the space between its halves. By connecting the movable pulley halves to a control unit, the pulleys in effect are always changing size, thereby constantly changing the speed ratio to permit optimal fuel consumption. This is not possible in conventional transmissions because they are limited to the few discrete gear ratios provided.
A main problem area still to be resolved is that of belt design. Rubber belts are preferred by some because of their low cost, their relative ease of replacement compared to metal belts, and the fact that they can be run dry as compared to steel belts which must be immersed in an oil bath. Their main drawback, however, is lack of durability. They are subject to fatigue failure caused by cyclical forces to which they are exposed and are also subject to fatigue failure caused by excess tension, resulting from attempts to prevent slippage between the belt and the pulleys.
Metal belts are favored by others because of their increased power capabilities, despite their high cost and lower efficiency. One design, for example, is a type of metal chain having links, much like in a bicycle chain, enabling it to be installed over the pulleys. Drive pins protrude from the sides of the chain and provide the necessary frictional contact between the chain and the pulleys.
In either case, traction between the belt and the pulleys is achieved by belt tension which, as pointed out above, is often excessive and contributes to belt failure. In addition, high belt tension puts extra high stress on the pulley bearings, causing them to wear out sooner than they should, and usually results in undesirably high operating temperatures, which tends to shorten belt like and reduces the efficiency of the transmission. Because the belt is subject to wear from its constant driving contact with the pulleys it is subject to eventual failure. The replacement cost of a belt is relatively high, more so for a metal belt, and the cost of the installation labor can be quite significant. Moreover, failure of the drive belt while on the road could be totally disabling, since it is unlikely that the driver would have a spare belt on hand or be able to install it himself if he did have one.
What is needed is an improved arrangement which lessens the likelihood of drive belt failure and is not itself expensive. It would be desirable in addition to have such an arrangement which increase the coefficient of friction between the belt and the pulley, thereby eliminating the need for excessive belt tension as a means for providing adequate driving contact between the belt and the pulleys.